The extrusion of thermoplastic materials, as presently practiced, is based on the technique developed by Bewley in the 1840's. Essentially, the process involves feeding granular thermoplastic material continuously from a hopper to a heated cylinder in which an archimedean screw moves the softened polymer and forces the polymer through a die of the desired profile. During transport, the temperature of the plastic material is increased both by contact with the hot walls of the heated cylinder or barrel and by frictional heat developed during working. Kirk-Othmer, "Encyclopedia of Chemical Technology," II, Interscience publishers, New York, Vol. 15 (1968) at 805.
In the fabrication of plastic bodies, it is common to add fillers. Fillers are inert, generally low-cost, materials added to resinous bodies to make the resulting product less expensive or to effect other desirable properties. Fillers generally provide several advantages, including increased strength and stiffness, cost reduction, decreased shrinkage, improved wet strength properties, attractive color or appearance, higher heat resistance, and the like. On the other hand, addition of fillers to a plastic matrix may impose limitations on the mode of fabrication used, inhibit curing reactions of thermoset resins, decrease storage life of the resins or increase the density of the product.
Fillers are usually granular or particulate inorganic material, although fibrous fillers are sometimes used. Among fillers used in large volumes in reinforced plastics are alumina, asbestos, chalk, calcium silicates, glass beads, graphite or carbon black, iron oxides, magnesium oxide, mica, silica, titanium dioxide and clays, such as kaolin and montmorillonite. Kirk-Othmer, supra, Vol 12 (1967) at 191. The cost of fillers such as clay is as low as 1 cent per pound whereas a high quality magnesium oxide filler costs of the order of 28 cents per pound and would be used sparingly, especially in relatively inexpensive resins, unless its use were essential for some reason.
The use of nutshell flours as fillers is disclosed broadly in Kirk-Othmer, supra, Vol. 14 (1967), at 142. For example, walnut-shell flour has been used in radio horns and loudspeakers. Also, floor coverings have been developed from a mixture of nutshell flour, water-insoluble aluminum, pigment and resins. The material is applied to a flexible base and then covered with coloring and binding material to fill any pores or crevices.
With respect to curable polyesters, such as semi-flexible resin containing 90% by weight of Cyanamid EPX-279-1 and 10% by weight of EPX-187-3, with 25 parts per hundred of added styrene, optimization data have been developed for inclusion of certain types of fillers, more specifically, clay, marble, pecan shell flour and glass microballoons. W. C. Jones, III, "Optimization of the Filler Content of Filled Polyester Resin," J. Applied Polymer Sci., Vol. 15 (1971) at 1109. As shown from the following material compiled by Jones, nut shell flour is competitive economically with clay and certain types of marble fillers:
______________________________________ Density and Costs of Resins and Fillers ______________________________________ Material Density Cost Cost ______________________________________ g/ml $/lb. $/liter Polyester resin 1.2 0.25 0.666 I.G. 101 Micro- balloons 0.34 0.69 0.517 Nut Shell Flour 1.30 0.04 0.114 Marble Gamma-Sperse 255 2.71 0.012 0.072 RO-40 2.71 0.005 0.030 P4-40 2.71 0.005 0.030 Clay Hydrite 10 2.58 0.026 0.148 Hydrite Flat D 2.58 0.014 0.080 Hydrite MP 2.58 0.015 0.085 ______________________________________
However, the prior art does not teach the use of dried nut shell flour as a filler or extender containing less than 5% water for extrudable thermoplastic polymers, especially polyamides and thermoplastics derived from ethylenically unsaturated monomers, e.g., polystyrene, polyethylene and polypropylene.